This invention relates generally to papermaking and more particularly to a shoe press belt, for use in a papermaking machine, having a superior water draining effect, and to a method of manufacturing the belt.
Shoe press devices adopted for use in the press stage of a papermaking process in recent years may be roughly divided into two types. One is shown in FIG. 8, and another is shown in FIG. 9. In both of these shoe press devices, a shoe 62 is in opposed relationship with a roll 61, with upper and lower endless felts 63 and 64 provided between the shoe and the roll, and a wet web P therebetween. A press belt 65 is arranged between the lower felt 64 and the shoe 62 so that the press belt 65 runs along with the lower felt 64. The shoe 62 raises the press belt 65, thereby pressing the felts 63 and 64 against the roll 61. Thus, a relatively wide nip area is formed and water squeezing is effected by the pressure between the roll 61 and the shoe 62.
The press belt 65 of FIG. 8 is a comparatively long belt, spanning a plurality of rolls 66, there being four such rolls in the particular shoe press device depicted in FIG. 8. The press belt 65 is adapted to run under tension. On the other hand, the press belt 65 of FIG. 9 is a comparatively short belt.
As shown in FIG. 10(a), the press belt 65, used for the two types of shoe press, is generally composed of a base member 65a sandwiched by a wet web side layer 65b and a shoe side layer 65c, both of which layers are composed of high molecular weight elastic members. The surface of the high molecular weight elastic member 65b is either a flat surface H as shown in FIG. 10(a), or has a grooved water-holding section M as shown in FIG. 10(b).
The press belt 65, having a flat surface H as shown in FIG. 10(a), may be completed at low cost, since only grinding the wet web side is necessary in the manufacturing process. The low manufacturing cost is the reason why this type of press belt is still in wide use. On the other hand, in the use of the press belt 65 of FIG. 10(b), having a water-holding section M, the water squeezed from the wet web P (FIGS. 8 and 9) by the pressure applied by the roll 61 and the shoe 62, is retained within the water holding section M, so that the water squeezing efficiency of the belt of FIG. 10(b) is far greater than that of the belt of FIG. 10(a). Unexamined Japanese Utility Model Publication No. 54598/1984 is representative of the belt having a water-holding section. In this case, a material having a hydrophilic property, such as polyurethane resin, is used as a high molecular weight elastic material.
Notwithstanding the improved water squeezing efficiency afforded by the press belt of FIG. 10(b), the amount of moisture which remains in the belt has increased as result of the use of increased nip pressures and greater operating speeds in recent years, and this moisture retention has been an obstacle to water squeezing efficiency improvement. That is, when the nip pressure of the roll 61 and shoe 62 is increased, more water is squeezed from the wet web, but the result is that more water is held on the flat surface H (FIG. 10(a)) or the water holding section M (FIG. 10(b)) of the press belt 65. Therefore, in some cases, because of the strong affinity of the press belt surface for moisture, resulting from hydrogen bonding, when the press belt is made hydrophilic as taught in Unexamined Japanese Utility Model Publication No. 54598/1984, water may not be shaken off adequately from the press belt 65 in the tangential direction.
Under the nip pressure in such a situation, because of the moisture saturation in the felts 63 and 64, and in the press belt 65, it has not been possible to drain water effectively from the wet web. The tendency of the belt to retain water has become more significant with the recent demand for higher speed operation in papermaking machinery. The underlying reason for the greater water retention at higher operating speeds is that the more rapid movement of the press belt 65 results in the shortening of the time interval between the successive compressions of given parts of the press belt 65 by the roll 61 and the shoe 62. Consequently, the time available for water to be shaken off a given area of the press belt 65 between compression cycles inevitably becomes shorter. This has become a particularly acute problem in the operation of the shoe press device of FIG. 9. Excessive water retention was not only a problem in the case of a press belt 65 having a water holding grooved section M, but was also encountered as a problem in the case of a press belt 65 having a flat surface H.
An object of this invention is to provide a belt for a shoe press, which is capable of solving the above-mentioned problems, thereby improving the water-squeezing function. Another object of the invention is to provide a novel method for the manufacture of such a belt.
To achieve the above-mentioned objectives, the shoe press belt in accordance with the invention is a shoe press belt in which a wet web side layer of a main body of the belt comprises a high molecular weight elastic material, characterized in that the surface of the wet web side layer is hydrophobic. Consequently, water squeezed from the wet web under compression in the shoe press device, and shifting to the surface of the wet web side layer of the main body of the belt through the felt, may be shaken off reliably before the belt is again subjected to compression.
If the main body of the belt also comprises a water holding section on the surface of the wet web side layer, both the surface of the wet web side layer and at least a part of the water holding section are preferably hydrophobic. Thus, the moisture which is squeezed from the wet web under compression in a shoe press device, passed through the felt, and held on the surface of the wet web side layer of the main body of the belt, and in the water holding section, may be shaken off reliably before the belt is again is subjected to compression.
In another embodiment of the invention in which a water holding section is provided on the surface of the wet web side layer of the belt, the surface of the wet web side layer may be hydrophilic, but at least a part of the inner surface of the water holding section is hydrophobic. In this case, moisture which is squeezed from the wet web under compression in the shoe press device, passed through the felt, and held on the surface of the wet web side layer of the main body of the belt, may be shaken off reliably by virtue of the hydrophobic property of the water holding section before the belt is again subjected to compression.
Preferably, the hydrophobic property is such that the contact angle between a drop of water and a reference plane corresponding to the surface of the belt is at least 50xc2x0, thereby enhancing the effect of the hydrophobic property of the surface of the wet web side layer, or of the water holding section, in promoting shaking of moisture off the belt.
The belt is preferably manufactured by forming a wet web side layer of a main body of the belt with a high molecular weight elastic material having a hydrophobic property, and forming a hydrophobic surface by grinding the surface of the wet web side layer. Thus, a surface having a hydrophobic property may be easily produced on the wet web side layer of the main body of the belt.
The method of manufacture may optionally include a third step, in which a water holding section is formed on the surface of the wet web side layer. Thus, both the surface of the wet web side layer of the main body of the belt and the inner surface of the water holding section, can be easily made hydrophobic.
In an alternative method, a wet web side layer of the main body of the belt is formed of a high molecular weight, hydrophobic elastic material, a film comprising a high molecular weight elastic material of hydrophilic property is formed on the surface of the wet web side layer, and a water holding section is formed, extending inward from the film. In this manner, it is easy to make only the inner surface of the water holding section hydrophobic.
In accordance with still another alternative method, a wet web side layer of the main body of the belt is formed of a high molecular weight, hydrophilic elastic material, a water holding section is formed on the surface of the wet web side layer, and a film comprising a high molecular weight, hydrophobic, elastic material is formed on an inner surface of the water holding section. In this manner, it is easy to make only the inner surface of the water holding section hydrophobic.